Dozens of diseases can cause hair loss, ranging from autoimmune conditions and thyroid disorders to nutritional deficiencies and infections. The type of hair loss, whether it’s patchy or diffuse, sudden or gradual, reversible or permanent, often points directly to the underlying cause. Understanding these patterns can help you recognize when hair loss signals something that needs medical attention.
Autoimmune Conditions
Alopecia areata is the most well-known autoimmune cause of hair loss. It happens when your immune system mistakenly attacks your own hair follicles. Healthy hair follicles normally maintain a kind of protected status that keeps immune cells from interfering with hair growth. In alopecia areata, that protection breaks down. Immune cells flood the follicle, release inflammatory signals, and force hairs out of their active growth phase prematurely. The result is smooth, round bald patches that can appear on the scalp, beard, eyebrows, or anywhere on the body. In severe cases, it progresses to total scalp hair loss or even complete body hair loss.
Lupus causes hair loss through two distinct pathways. The non-scarring type shows up as diffuse thinning and fragile hair across the scalp, and it’s generally reversible once the disease is controlled. Discoid lupus, a form that targets the skin, is more serious for the scalp: it causes scarring alopecia in about 20% of men and 50% of women with the condition. In scarring alopecia, the hair follicle is replaced by fibrous tissue, and once that happens, the hair loss is permanent and irreversible. You can tell the difference on close examination because scarring alopecia destroys the follicular openings on the scalp, while non-scarring types leave them intact.
Dermatomyositis, an inflammatory muscle disease, can also trigger diffuse hair shedding. And pemphigus vulgaris, a blistering autoimmune condition, sometimes causes erosive changes on the scalp that damage hair follicles.
Thyroid Disorders
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can disrupt hair growth. Thyroid hormones regulate metabolism throughout the body, including in hair follicles. When production swings too high or too low, a larger percentage of hairs shift into the resting phase at the same time. This is called telogen effluvium, and it shows up as excessive shedding during washing, brushing, or styling rather than distinct bald patches.
The hair loss from thyroid disease is typically diffuse, meaning it thins evenly across the scalp rather than in one spot. The good news is that it’s usually reversible. Once thyroid hormone levels are brought back into a normal range, hair gradually returns to its regular growth cycle over several months. If you’re noticing increased shedding along with fatigue, weight changes, or feeling unusually cold or warm, thyroid dysfunction is worth investigating.
Iron Deficiency and Nutritional Gaps
Iron deficiency is one of the most common and most overlooked causes of hair loss, especially in women. Your body needs stored iron to support hair follicle activity, and the threshold for hair health is higher than what standard lab reports flag as abnormal. Most labs consider ferritin (the protein that reflects your iron stores) normal at 15 to 30 ng/mL, but dermatologists and hair specialists use a different scale:
- Below 30 ng/mL: highly likely to contribute to hair loss
- 30 to 40 ng/mL: possibly too low for optimal hair growth
- 40 to 70 ng/mL: minimum range for healthy hair
- Above 70 ng/mL: optimal for hair growth
This means you could have “normal” iron levels on a standard blood test and still be losing hair because of low ferritin. Profound iron deficiency anemia causes diffuse hair shedding that looks similar to thyroid-related loss.
Zinc deficiency, whether from poor absorption, long-term IV nutrition, or an inherited condition, can trigger severe telogen effluvium as well. Conditions that impair nutrient absorption, such as celiac disease, inflammatory bowel disease, and liver or kidney disorders, can all cause hair loss indirectly by depleting the vitamins and minerals your follicles need.
Fungal and Bacterial Infections
Ringworm of the scalp (tinea capitis) is a common fungal infection, particularly in children. The fungus invades the outer layer of scalp skin and the hair shaft itself, causing hairs to become brittle and break off at or near the surface. The telltale signs are round, scaly patches with small black dots where hairs have snapped off. The scalp may look silvery and feel tender or painful.
In more severe cases, the infection triggers a kerion, a raised, spongy mass on the scalp that drains pus and develops thick yellow crusting. Hair in the affected area falls out or pulls away with almost no resistance. Ringworm-related hair loss is reversible with treatment, though a kerion can sometimes leave a small area of scarring.
Secondary syphilis, a bacterial infection, occasionally causes a distinctive pattern called “moth-eaten” alopecia, with irregular patchy hair loss scattered across the scalp. This occurs in roughly 4% to 7% of secondary syphilis cases. The hair loss is non-scarring and resolves with treatment of the underlying infection.
Hormonal Imbalances Beyond the Thyroid
Excess androgens (male-pattern hormones present in all genders) are a major driver of hair loss. In androgenetic alopecia, the most common form of hair loss worldwide, hair follicles on the scalp gradually miniaturize in response to these hormones, producing thinner, shorter hairs until growth stops entirely. While androgenetic alopecia has a strong genetic component, elevated androgen levels from conditions like polycystic ovary syndrome (PCOS) can accelerate or worsen the process.
The postpartum period is another hormonally driven trigger. During pregnancy, high estrogen levels keep more hairs in their growth phase than usual. After delivery, those hormones drop sharply, and a large wave of hairs enters the resting phase at once, leading to noticeable shedding that typically peaks two to four months after birth. This is temporary and resolves on its own.
Severe Illness and Physical Stress
Any major physical stressor can push large numbers of hair follicles into a resting state simultaneously. High fevers, severe infections (including HIV), major surgeries, extreme dieting, and rapid weight loss are all established triggers for telogen effluvium. The shedding usually doesn’t start until two to three months after the triggering event, which is why people often don’t connect the two.
This type of hair loss is almost always temporary. Once the body recovers from the stressor, the hair growth cycle resets. Full regrowth typically takes six to twelve months, though it can feel alarming in the meantime because the shedding can be dramatic.
How Doctors Identify the Cause
When hair loss points to an underlying disease, doctors typically start with a physical exam of the scalp. The pattern of loss, whether follicular openings are still visible, and the condition of the remaining hair all offer clues. Smooth round patches suggest alopecia areata. Diffuse thinning with no scarring points toward thyroid issues, nutritional deficiencies, or hormonal imbalances. Scarred, shiny patches where follicles have been destroyed raise concern for discoid lupus or other scarring conditions.
Blood tests narrow the diagnosis further. A standard workup for unexplained hair loss often includes ferritin levels to check iron stores, thyroid-stimulating hormone (TSH) to evaluate thyroid function, and androgen levels if hormonal excess is suspected. If an autoimmune condition is on the table, additional testing may follow. The specific combination of tests depends on the pattern of hair loss and any other symptoms you’re experiencing.
Scarring vs. Non-Scarring Hair Loss
The single most important distinction in disease-related hair loss is whether the follicle itself has been destroyed. Non-scarring hair loss, which includes telogen effluvium, alopecia areata, and most nutritional or hormonal causes, leaves the follicle intact. Hair can grow back once the underlying problem is addressed. Scarring alopecia, caused by conditions like discoid lupus, certain blistering diseases, or severe infections, replaces the follicle with scar tissue. That hair is gone permanently.
This is why early evaluation matters. In scarring conditions, treatment aims to stop the process before more follicles are lost. In non-scarring types, identifying and correcting the root cause is often enough to restore growth over time.